1. Field of the Invention
The invention relates to the fields of mechanical engineering and chemistry and relates to long-term stable oil polytetrafluoroethylene (PTFE) dispersions, such as can be used, for example, in moving parts with tribological requirements in terms of sliding friction and wear, such as, for example, in transmissions or bearings, and a method for the production thereof.
2. Discussion of Background Information
A great number of oil-PTFE dispersions are described. A dispersion of PTFE in oil led in a short time, with as well as without additives, to sedimentation of the PTFE, which then can usually be fluidized/redispersed only with great difficulty.
However, very many publications deal only with the composition of the dispersions or with the use of special additives for better stabilization of the dispersion, the focus not being on the dispersion stability.
From FR 2857373 A1 a PTFE-containing mineral oil is known with 80 to 97 (preferably 95 to 97) % by weight of mineral oil and 0.5 to 1.5 (preferably approx. 1) % by weight of submicron PTFE, as well as perfume and dye additives, in the dispersion for domestic and workshop use.
Likewise known are the tests according to M. G. Ivanov et al., Ural. Gos. Tekh. Univ. UPI, Yekaterinburg, Russia. Trenie i Iznos (2004), 25(1), p. 99-103 on the tribological properties of a lubricant, comprising oil, PTFE and ultradispersed diamond, in a steel-on-steel friction couple.
Furthermore, according to JP 2003113390 A a method is known for dispersing PTFE powder (average particle diameter of 7 μm) in basic lubricating oil at ≦40° in an overpressure homogenizer and then in a ball mill to reduce the average particle size≦1 μm for the production of lubricating oil additives for diesel engine crankshaft housings.
These publications do not give any information on dispersion stability.
Furthermore, according to JP 11021577 A, a lubricant is known, which is composed of a base oil (1) selected from plant oils, animal fats, mineral oils, ester oils, mixed oils and modified oils and PTFE powder (2), which was dispersed in the base oil in the presence of a multivalent metal salt of a higher fatty acid, which is dissolved in the base oil.
Likewise known according to U.S. Pat. No. 5,846,447 is a process for forming PTFE dispersions in an organic carrier fluid (particularly lubricating oil), in which (1) the treatment of the PTFE particles diameter<1 μm (preferably <0.25 μm) with electron radiation or gamma irradiation of ≧4 Mrad in the presence of atmospheric oxygen and water, (2) the mixing of the irradiated particles with the lubricating oil and ≧1/at least one dispersant, and (3) the passage of this mixture through a plurality of nozzles within a liquid-jet interaction chamber takes place at a pressure of ≧1000 psi to deagglomerate and stabilize the PTFE particles. It is reported that the method yields a stable uniform PTFE dispersion and the introduction of undesirable water into the lubricant is avoided.
According to U.S. Pat. No. 5,744,539 A, furthermore a PTFE dispersion is known, which essentially comprises a stable dispersion of finely distributed PTFE particles and a polymeric dispersant in a lubricating oil, in which the dispersions are essentially free of a PTFE sedimentation.
According to PL 168489 B1 a method for producing PTFE suspensions is described, which are used as additives for lubricants and hydraulic oils. A 16% by weight aqueous PTFE suspension (containing PTFE 60, ammonium perfluorocaprylate 0.02, nonionic surfactants 0.02, etc.) is added to lubricating oils and hydraulic fluids. In the further production process 0.2 to 4% by weight dispersant, 0.3 to 4% by weight surfactant and 20% by weight thickener are also added to the oil mixture.
From NL 9300742 A a lubricant is known, based on a solid resin (Teflon/PTFE), which is dispersed in a carrier in the lubricant. The carrier substance is silicone oil; the dispersing is carried out in the presence of a dispersant.
Furthermore, a method is known from DE 36 42 617 C1 for producing a PTFE dispersion, which is mixed in oil as lubricating oil or its additives, in which PTFE in powder form or as an aqueous dispersion in the presence of nonionic surfactants (antistatic agents) in a commercial mineral oil.
According to DD 25 19 88 A1 a PTFE-containing lubricant is known, which as a dispersant contains polyfluorocarboxylic acids and esters thereof, which improve the stability of the dispersed PTFE powder with particle sizes of 0.1 to 0.5 μm.
H. Driescher et al., Schmierungstechnik (1984), 15(7), 199-202, describe the active service life of a precision lubricant for use in high vacuum, in which fine oil is used, which contains PTFE fine powder and, to stabilize the PTFE dispersion, perfluoroalkenyl alkyl ether dispersant.
According to U.S. Pat. No. 3,933,656 A a lubricating oil with fluorocarbon additive with boundary layer properties and with a reduced emission is known, in which 300 cm3 submicron-size PTFE [9002-84-0] powder and 30 cm3 silane A 1100 [919-30-2] is dispersed in the same amount of motor oil.
It is known from all of these disclosures that a certain dispersion stability can be achieved at all or can be increased only through the addition of additives.
Furthermore, according to RU 2212418 C1 a dispersion of ultrafine PTFE is known, produced by thermal decomposition of PTFE at 480 to 540° C. in the presence of an oxygen-containing compound that is thermodynamically suitable for PTFE oxidation, under the atmosphere of the gaseous degradation products. The degradation products/decomposition products from the degradation process are cooled and condensed during passage through a mineral oil, whereby a PTFE dispersion (12.9 g dry PTFE with an oxygen content of 1.7%) is formed. The ultrafine PTFE formed is lyophilic and forms stable dispersions without the addition of stabilizers or surfactants.
The described gaseous oxidized degradation products of the PTFE are thereby in fact no longer PTFE, since they no longer exhibit the advantageous properties of PTFE to the full extent.
From U.S. Pat. No. 4,465,607 A furthermore a PTFE-containing lubricant composition is known, in which the PTFE with particle sizes of 1 to 200 μm produces stable dispersions through ultrasonic agitation/treatment in a base lubricating oil. No information is provided on the long-term stability.
S. Palios et al., Tribology Series (1996), 141-152, Publisher: Elsevier, tested the behavior of PTFE suspensions (PTFE in lubricating oil) as a function of the PTFE particle size in rolling/sliding contact. Larger PTFE particles appear to reduce friction and wear. It was not disclosed whether the particles adhere strongly to the rubbing side and form a permanent coating. Very small PTFE particles in completely formulated oils do not appear to make any measurable contribution to the reduction of friction and wear.
The principle of reactive coupling/surface modification of PTFE nano/micro powder particles with olefinically unsaturated monomers/compounds is already known from DE 103 51 812 A1, DE 103 51 813 A1 and DE 10 2004 016 876 A1.
The disadvantages of the known solutions are that no long-term stable oil PTFE dispersions are known.